A large fraction of PsaF is nonfunctional in photosystem I complexes lacking the PsaJ subunit

PsaJ is a small hydrophobic subunit of the photosystem I complex (PSI) whos e function is not yet fully understood. Here we describe mutants of the gre en alga Chlamydomonas reinhardtii, ill which the psaJ chloroplast gene has been inactivated either in a wild-type or in a PsaF-deficient nuclear backg round. Cells lacking one or both subunits grow photoautotrophically and con tain normal levels of PSI. Flash-absorption spectroscopy performed with iso lated PSI particles isolated from the PsaJ-deficient strain indicates that only 30% of the PSI complexes oxidize plastocyanin (Pc) or cytochrome c(6) (Cyt c(6)) with kinetics identical to wild type, whereas the remaining 70% follow slow kinetics similar to those observed with PsaF-deficient PSI comp lexes. This feature is not due to partial loss of PsaF, as the PsaJ-less PS I complex contains normal levels of the PsaF subunit. The N-terminal domain of PsaF can be cross-linked to Pc and Cyt c(6) indicating that in the abse nce of PsaJ, this domain is exposed in the lumenal space. Therefore, the de creased amount of functional PsaF revealed by the electron-transfer measure ments is best explained by a displacement of the N-terminal domain of PsaF which is known to provide the docking site for Pc and Cyt c(6). We propose that one function of PsaJ is to maintain PsaF in a proper orientation which allows fast electron transfer from soluble donor proteins to P700(+).